In extruding systems for plastic parts, in particular plastic pipes, a refrigerating segment comprised of several cooling chambers is arranged in back of the extruder with its extruder implement viewed in the direction of production. The inlet of the first cooling chamber under a vacuum is formed by a calibrating shell, in which the external dimensions of the plastic part are calibrated to the specified size. The excessively large plastic part exiting the extruder element in a warm, and hence still formable state is reduced to the specified size in the calibrating shell. This is accomplished by the vacuum in the cooling chamber draws the part, in particular the plastic pipe, to the interior wall of the calibrating shell via the holes provided in the calibrating shell.
In order to produce plastic parts with varying external dimensions, in particular external diameters, in one and the same extruding system, it is known either to replace the calibrating shell or use a calibrating shell with an infinitely adjustable diameter. However, circular cross sections cannot be achieved with most known infinitely adjustable calibrating shells, since the calibrating shell consists of segments. In addition, the adjustment range of such calibrating shells is comparatively small (DE 198 43 340 A1, DE 200 00 872 U1, DE 200 23 052 U1, EP 11 57 805 A1).
Another known infinitely adjustable calibrating shell (DE 44 08 064 C1) makes it possible to achieve an absolutely circular cross section of the extruded part. In this calibrating shell, the middle section consists of bands wound in the form of a two-start thread with a high pitch. Turning both ends of this middle section toward each other permits only a variation of its internal diameter within narrow limits. The windings of the band are secured against lateral deflection by support rails arranged parallel to the calibrating shell, preferably having a flexible design in the circumferential direction of the calibrating shell. Such support rails limit the adjustment path of the calibrating shell. Another disadvantage is that the calibrating shell lacks a high intrinsic stiffness, so that the terminal annular heads absolutely must be connected with each other by several supporting pipes.